Most of automotive members are required to have a high structural strength. This requirement can be satisfied by various methods such as a selection of the material from a high tension steel sheet or an extra high tension steel sheet that can perform over 590 N/mm2, an increase of the sheet thickness of the automotive member, an addition of the reinforcement members, or the like. Alternatively, the high structural strength can also be realized by hardening the automotive member manufactured by a steel sheet having tension in a range of 270 N/mm2 to 440 N/mm2 which is in good value for cost together with superior workability.
A door beam can be exemplified to the automotive member as further required to be light weight in addition to the high structural strength, low cost and high workability, as described above. The door beam disclosed in Japanese Patent No. 3,139,984, for example, comprises three members, i.e., a body portion and a pair of mounting portions (or mounted brackets) It discloses that the mounting portions are integrally welded to the both ends of the body portion, and are subjected, if necessary, to a thermal treatment such as hardening or annealing operation (as referred to Column 76 of the Citation).
JP-A-10-166860 discloses the door beam formed by press molding from a single member so as to intend to lower the cost for the thermal treatment such as the hardening or annealing operation. Further, in order to prevent a residual stress occurred in the aforementioned thermal treatment, it has selected the high tension steel requiring no thermal treatment such as the hardening or annealing operation (as referred to Column 19 or others of the Citation). On the contrary, JP-A-2003-094943 takes the issue of a thickness increase and a weight growth of the sheet both caused by intending to keep its structural strength, instead it does not adopt the thermal treatment such as the hardening or annealing operation. Thus, as describing in the specification of JP-A-2003-094943, it discloses a door beam, which is enabled to satisfy the necessary structural strength by thermally treating the molded sheet having a thin thickness into hardening or annealing operation although the door beam is made from a single member as in the aforementioned door beam of JP-A-10-166860.
In order to design for use in a continuous production line, on the other hand, JP-A-09-002066 adopts an electromagnetic induction heating, i.e., a high-frequency hardening operation which can perform a thermal treatment such as the hardening and annealing operations while moving the press-molded door beam along the production line. In particular, it discloses that a door beam is hardened in high frequency only at its body portion defined as an intermediate portion of the door beam required to be improved its structural strength, thereby to satisfy demands for higher structural strength, cheaper, easier workability and lighter weight (as referred to Column 7 or others of the Citation).
In a batch treatment, if it is sufficient, a direct current can be applied as a means for hardening the door beam. JP-B-44-014455 discloses a thermal treatment method for instantaneously hardening and annealing the body to be heated having a long shape like a door beam by applying a pulse current directly to the body. In this JP-B-44-014455, the heating by applying a pulse (AC) current controls the pulse width capable of realizing the instantaneous hardening and annealing operations by considering the “skin effect” caused in the area of the surface of the body to be heated.